Deep-water Earliest Oligocene Glacial Maximum (EOGM) in South Atlantic

ARTICLESChinese Science Bulletin 2004 Vol. 49 No. 202190 - -2197cene/Oligocene boundary at 33.7 Ma. This event is calledthe significant phase of the global climate shift in theDeep-water Earliest OligoceneEarth's history". Recent high-resolution benthic fo-Glacial Maximum (EOGM) in rainierel isope rrs in wrdvide 0pes idiaiethat the rapid δ 18O increase of 1.4 %oin the earliest Oli-South A tlanticgocene (33.6 Ma), occurring within ~400 ka, marks therapid expansion of permanent continental ice sheets oneast Antarctica and a minimum of 3- 4.( of cooling ofLIU Zhifei, TUO Shouting, ZHAO Quanhong,CHENG Xinrong & HUANG Weibottom waters. This period with the first appearance ofpermanent ice sheets on Antarctica was called the EarliestLaboratory of Marine Geology, Tongji University, Shanghai 200092,Oligocene Glacial Maximum (EOGM) or 0i-1 event2 .Chinaoccurrence of global EOGM event,Correspondence should be addressed to Liu Zhifei (e- mail: lzhifei@critical components of the Earth system including seaonline.sh.cn)level, atmospheric CO2, land/sea surface coverage, conti-Abstract The most prominent cooling event of the Earthnental weathering rates, and ocean chemistry， each ofsurface during Cenozoic in the long-term transition from awhich has the potential to trigger large-scale physical andnon-glaciated planet, or“green-house world", to a polar,geochemical feedbacks, changed significantly4. More-glaciated planet, or“ice-house world", is the Earliest Oligo-cial Maxirimum EOGM) above the Eocene/Oligocene;over, the EOGM event is the rapid climate change, whichboundary at about 33.7 Ma. Planktonic and benthic fo-has closed relationship to instantaneous changes in at-raminiferal oxygen and carbon isotopes, carbonate content, mospheric CO2. The variation rates are close to the mod-and coarse fraction, along with high-resolution color reflec- ern, atmospheric CO2 changes caused by human activi-tance and magnetic susceptibility records during 35. 30 Ma，ties5s!. Therefore, the further study on the EOGM event infrom deep-water Sites 1262 and 1265, Ocean Drilling Pro-sediment records will contribute to the understanding ofgram (ODP) Leg 208 in South Atlantic, reveal the globalthe past global climate change on Earth and to the fore-cooling event occurring in both surface and deep oceans. Thecasting of climate change in future.results show that the earliest Oligocene δ 180 values duringThe best materials to conduct the EOGM event study33.5 - 33.1 Ma represent the magnitude of continental iceure continuous, non-disturbed, and diagenesis-ignoredsheets on east Antarctica and indicate the large decrease inboth surface and deep water temperatures of worldwidedeep-sea sediments, which are distributed in worldwideoceans. The δ“C records show the large excursion duringoceans. Those sediments have been collected by cruises ofthe period of EOGM event and indicate some types of shift inthe Deep Sea Driling Program (DSDP) and Ocean Drill-global carbon reservoir, probably demonstrating the sudden ing Program (ODP). Various excellent studies have beenincrease in organic carbon burial rates and the changes in performed, including many planktonic and benthic fothe distribution and timing of production. At the same time, raminiferal oxygen and carbon isotopelithologic composition, carbonate content, color rllectance, However, the previous studies were based mainly on sitesand coarse fraction brought about significant changes closein high-latitude, near Antarctica and only a few of themto the Eocene/Oligocene boundary, reflecting the abruptdealt with sediments with different water depths at thedeepening in the carbonate compensation depth (CCD).same/closed sites, especially in low-latitude oceans. InChanges in carbonate content were revealed from the coloraddition, because of drilling technique reasons during ear-reflectance identify periodicities associated with eccentricitylier DSDP and ODP periods, the core recovery was gener-of the Earth's orbit (100 and 400 ka), further indicating or-bitally forced global climate variations in the Early Oligo-ally less than 60%- 70%9), thus constraining high-reso-cenelution study on climate changes.In 2003, the ODP Leg 208, titled“Early CenozoicKeywords: Earliest Oligocene Glacial Maximum (EOGM), Eocene/Extreme Climates", collected early Cenozoic sedimentsOligocene boundary, South Atlantic, stable isotopes, Ocean DrillingProgram (ODP).from six sites with water depths between 2500 and 4700DOI: 10.1360/04wd0228m on the northeast W alvis Ridge in South Atlantic. Five ofall sites used the triple Advanced Hydraulic Piston CorerSince the beginning of Cenozoic, Earth' s climate has (APC) to retrieve the EOGM sediments and ensure theundergone the continuous cooling evolution with various ignored dilling disturbance and the 100% recoverylo.extremes of cold with massive continental ice-sheets, fromBased mainly upon oxygen and carbon stable isotopes ofa non-glaciated planet, or“green-house world"', to a polar, planktonic中国煤化工rbonate content,glaciated planet, or “ice -house world’. The long-term coarse fractioFlagnetic suscepti-transition from “green-house" to“ice-house" conditions bility of ODFMYHCNMHGninwaterdepth)occurred during the middle Eocene to early Oligocene.and on associated benthic foraminiferal oxygen and car-Although this interval encompasses as long as 18 Ma, thebon stable isotope records of Site 1265 (3060 m in watermost prominent cooling event happened close to the Bo- depth), this study reveals the global climate changes dur-21月方数据Chinese Science Bulletin Vol. 49 No. 20 October 2004ARTICL ESing the Eocene-Oligocene transition (35- 30 Ma) in nannofossil and fragments of foraminifers. Because thedeep-water South Atlantic Ocean.water depth of this site during the Eocene-Oligocene tran-sition period is much close to the CCD, the content of1 Materials and methodsSite 1262 (27911.15 S 1° 34.62 E) is situated in theforaminifers, especially benthic species, is very low andthe planktonic species mainly consists of fragments,Angola Basin, northwestward the Walvis Ridge in Southwhich theoretically belong to anti-dissolution species. It isAtlantic Ocean (Fig. 1), with a water depth of 4755 m,impossible for most of them to be further identified. Thewhich is close to the modern carbonate compensationcarbonate content of the Eocene interval is generally <5%depth (CCD) of about 5.0 km in the eastern Atlantic. Theand almost ncalcareous foraminifer exists in sedi-CCD is a significant sedimentary boundary in deep seaments' 0. Therefore, fragments of planktonic foraminifersfrom calcareous to non-calcareous sediment distributions.in the Oligocene interval (68.86 - 78.87 mcd) are meas-Above the depth, the amount of carbonate sediments de-ured for oxygen and carbon stable isotopes. Fragmentsposited from overlaid water column keep a balance withwith sizes of > 150 μm were selected randomly to yielddissolved carbonate; whilst below the depth, carbonateaverage isotope compositions of anti-dissolution plank-contents are generally <10% and even dissolved com-tonic foraminifers and to avoid specific isotopic vital ef-pletely". The CCD in South Atlantic holds at such depthfects. The isotope compositions obtained from this methodfor most of the Cenozoicl121. Site 1265 (28° 50.10 S,are generally not as good as to results from single species,20 38.35 E) with a water depth of 3060 m is located on thebut much better than those of bulk samples, while the lat-Walvis Ridge (Fig. 1). Along with Site 1262, they consti-ter is usually used in the international paleoceanogra-tute a deep-sea transact.phy13].Total 134 samples were collected between 68.84 andTotal 122 samples were collcted between 167.0387.64 mcd (composite depth meters) from Site 1262. Theand 199.96 mcd from Site 1265, with a depth resolution ofaverage 7.5 cm sampling interval for most part of the sec-27 cm and a temporal resolution of 50 ka. The Eocene/tion (72.98 - 80.95 mcd) yields a temporal resolution ofOligocene boundary is located at 191 .88 mcd. The site42 ka, with a higher temporal resolution of 10- 20 karecords gradual changes over a 40-cm interval from yel-during 33.0- 33.6 Ma interval. The Eocene/Oligocenelow-brown clay-bearing nannofossil ooze in the upper-boundary is located at 78.87 med. The site records abrupt most Eocene to gray foraminifer bearing nannofossil ooze .lithologic changes over a <10-cm interval in the upper-in the lowermost Oligocene. Carbonate content throughoutmost Eocene and lowermost Oligocene, from brown claythe studied interval is around 85% - 92%n0. They mainlybelow to light gray foraminifer- bearing nannofossil oozeconsist of abundant foraminifers and calcareous nannofos-above. Carbonate content of the Oligocene interval is sil and ensure the measurement of foraminiferal oxygenaround 70% - 90%，but mainly consists of calcareousand carbon isotopes.20"S25 "SCape BasinDSDP522ODPI262好400 Angola BasinODP126530°S中国煤化工s°w0°E5°EYH10"ECNMH GFig.1. Bathymetric chart (in meters) of Walvis Ridge in South Atlantic Ocean, showing locations of Sites 1262 and 1265.Ch序野数rience Buletin Vol.49 No. 20 October 20042191ARTICLES .Fragments (<>150 μm) of planktonic foraminifers 0.7 %oand 2.2 %oand reach a maximal peak with an in-from Site 1262 and benthic foraminifer C. mundulus crease 0.9 %oat ~33.4 Ma in the early Oligocene. Those(>150 μum) from Site 1265 were selected and cleared. features coincide with the global information revealed byTheir oxygen and carbon isotope analysis was conductedother planktonic foraminifers7. The Site 1265 displaysusing a Finnigan MAT 252 mass spectrometer at the complete δ8O andδ "C records for the 35-30 Ma in-Laboratory of Marine Geology, Tongji University. Data terval, revealed by the benthic foraminifer C. mundulus.were reported relative to the Pee Dee belemnite standardThe Eocene/Oligocene boundary at 33.7 Ma and the shortwith an external error of less than +0.08 %o CarbonateEOGM event are obviously displayed (Figs. 3 and 4). Thecontent. was determined using the gasometric tech- δ 18O values vary in the range of 0.6 %- 2.4 %oand in-niques4, which have a precision of better than +2 %o crease rapidly from 1.2 %cto 2.4 %owithin <20 ka at 33.6Statistics of coarse fraction was taken from the ratio ofMa. The δ“C values vary between 0.1 %oand 1.9 %candcoarse particles (>63 um) with bulk sample to reflect reach maximal values during the EOGM event interval.various preservationls! Magnetic susceptibility and colorWe selected similar δl8O and δ I3C records of thereflectance (lightness, L*) were measured by onboard benthic foraminifer Cibicidoides spp. at DSDP Site 522automatic-controlled Bartington MS2 magnetic suscep-(266.84 S, 57.78 W, 4441 m water depth, Fig. 1) in thetometer and Minolta CM-2002 spectrometer, with a reso-Angola Basin!2), which is close to Site 1262, to comparelution of 2.5 cm, respectivelyto. We select the Morlet as a with our results. A good linear correlation for the upper-wavelet function' 16] to conduct the wavelet analysis after most Eocene to lowermost Oligocene interval is obtained,the linear interpolation of original magnetic susceptibilityi.e. higher values of the benthic foraminiferal δ 180 anddata and to discuss potential Earth' s orbital cyclicity.δ I3C of Site 522 correspond to higher δl8O and δ I3C val-We use the preliminary age-depth model decided af- ues of Sites 1262 and 1265, respectively, especially during .ter the cruise (Fig. 2)| 10]. The model was determined bythe EOGM event interval (Fig. 4).the onboard calcareous nannofossil and foraminifer strati-The abrupt increase in δ 18O values just above thegraphies, combined with the magnetic stratigraphy, withEocene/Oligocene boundary are recorded in world-widean average age error of +20 ka. Based upon the age-depthpelagic sediments. The excursion in high δ 180 values hasmodel, the average sedimentation rate for the 35. -30 Ma ben designated as 0i-1 or EOGM event2.18. The Sitesinterval is 0.26 cm/ka for Site 1262 and 0.57 cm/ka for1262 and 1265 records provide the same important aspectsSite 1265, respectively.of the event, the duration of roughly 400 ka and the profile160-τ6structure with two pronounced peaks of 0.7%o- 0.8 %omagnitude of increase, which keep the length of 100 - 450ODP Site 1265ka and are separated by a 150-ka minima (Fig. 4). The two昌170-F7o器δ 180 climax values, designated as Oi-1a and Oi-1b events,may represent the upper limit of the ice volume effect onseawater during the early Oligocene, or an ice sheet size,ODPSite 1262 F7slimit set by accommodation space of east Antarctical21. .The relatively low values between the two climaxes mayrepresent the“interglacial' stage ice volumes for that pe-190-▲Calcareous nannofossilsft 8riod. The termination of Oi-1 event occurred more slowly▲Planktonic foraminifersthan its initiation. The post-Oi-1 decline in isotopic valuesx Magmetostratigraphyinitiated at 33.2 Ma and continued to 32.9 Ma with severalbrief cooling events. After the termination, there was20293031323334353-85about 2-Ma period (31.0- 32.9 Ma) for the slow recoveryAge/Ma(Fig. 4). Previous studies mainly based on single site orFig. 2. Age -depth models of Sites 1262 and 126510.single water sitebut this study reveals the globalcooling climate event from both the deep-ocean floor,2 Results and discussionwhose water depth is close to the CCD, and the ocean(- 1) Foraminiferal oxygen and carbon stable isotopicsurface in South Atlantic, based upon a transect at thestratigraphies. The Site 1262 records in the early Oligo-same location with different water depths.cene interval show obvious trends of oxygen and carbonThe similarities in timing. and magnitude of thestable isotopes that are revealed by fragments of plank- Oi-1a/b δ 18中国煤化工and benthic fo-tonic foraminifers (Figs. 3 and 4). The δ 180 values vary in raminifers atIY片c N M H Guggest that thesethe range of0.5 %o 4.4%and display two maximal peaks events represewith increases of 0.7%o- 0.8%oin the early Oligocene and surface waters, i.e. an increase in ice volume and/or a33.1 - 33.5 Ma interval. The δ "C values vary between drop in sea-water temperature. Using the seawater/calcite21月方数据Chinese Science Bulletin Vol. 49 No. 20 October 2004Magnetic suscepibilityg"o (%u PDB)Caurbonale (%)(instrumcnt units)Fragments of8"O (%o PDB) C. mundulus02040608010050 100 150 200 250planktonic forauninifer0.5 10 1.5 20 2.50.0.5Coarse fraction {%)0.1上170723t 180g76EOGM190780.5 1.0 1.5 2.08"C (% PDB)IFragments of一+200304050607080ODP Site 1262planktonic foraniniler)5101.52Color ellectance L.* 1%)ODP Site 1265中国煤化工Fig. 3. Oxygen and carbon stable isotope records of fragments of planktonic foraminifers, carbonate content, coarse fraction, color reflectance (liceptibility of Site 1262, and oxygen and carbon stable isotope records of benthic foraminifer of Site 1265 plotted ve rsus depth.CNMHGfYH9Magnetic suscptibilityCurbonale (%)(instrument units)8"O(% PDB) C mmdhsδ"C(% PDB)C. mundulus20406080 10050 100 150 200 25030Coarse firaction (%)0.0.2 0.3言31 -32 .33OF1bEOGMO-10).s 1.00.5 1.0 1.s 2.034**O (%o PDB)δ"C (%o PDB)Fragments ofplanktonic foraminifer'plankonic foraminifer0.5 1.0 1.5 2.0 2.5 3.0I00.s0 1.5 2.0x ODP Site 1262台30 40506070808"O (% PDB) Cibicidoides spp,8*C (%0 PDB)Cibicidoides spp.ODP Site 1265Color rflcancc L* (%)DSDP Site 522Fig. 4. Oxygen and carbon stable isotope records of fragments of planktonic foraminifers, carbonate content, coarse fraction, color reflectance (lightnecg L*) nd mrratinsusceptibility of Site 1262, and oxygen and carbon stable isotope records of benthic foraminifer of Site 1265 plotted versus age for the 35- 30 Ma in中国煤化工stable isotope records of benthic foraminifer Cibicidoides spp. at Site 522 in the Angola Basin of South Atlantic2l were selected for their correlatio:fYHCNM HGARTICL ESequilibrium δ 180 equation!9), Zachos et al. (1996)21 ob- cene, and especially reach a maximum of ~90% during thetained a decline in bottom-water temperatures of ap- period of the EOGM event (Fig. 4). The color reflectanceproximately3- 4.( for the Oi-1 event at Site 522, whileL* also increases from ~44% in the late Eocene tosurface-water temperatures in world oceans decrease by50% - 70% in the early Oligocene, with a positive lineararound6- 8:Ccorrelation with variations in the carbonate contents, thus(-_ i) Early Oligocene carbon isotope xcursion andsuggesting the color reflectance can be served as an indi-global carbon cycle. The carbon isotope records fromrect proxy for the carbonate contentsk27. Variations inplanktonic foraminifer fragments of Site 1262 and benthicmagnetic susceptibility values, representing clay mineralforaminifera of Site 1265 show nearly identical long-termcontents，indicate an opposite relationship to both thetrends with those of Site 522 (Fig. 4). The most prominentcolor reflectance and the carbonate content, from ~200feature common to both records is the large excursioninstrument units in the late Eocene to ~ 50 instrument unitsduring the Oi-1 event, specially the maxima between 33.5in the early Oligocene (Fig. 4). The coarse fractions at Siteand 33.4 Ma, where values rapidly increase by 1.4 %oo1262 consist mainly of fragments of planktonic fo-Thus earliest Oligocene δ I3C excursion appears to beraminifers. The values are close to zero in the late Eoceneglobal in scale, supported by both benthic and planktonicand increase during the EOGM event period and reach itsrecords_ from the Pacific， Atlantic， and Indianmaximum of ~0.3% at ~33.0 Ma (Fig. 4). .The Eocene/Oligocene boundary is marked by one ofOceanst0-2. The excursion may increase as high as thethe largest known shifts at the level of the CCD during themaximal value of 2.0 %Z31. The occurrence of δ I3C excur-Cenozoic. During that time, the CCD deepened by assion coincides with the Oi-1 event. Our study further sug-much as 1 km in the equatorial Pacific and by about 0.5gests a cause and effect relationship.Changes in the ocean δ BC composition of suchkm in the Atlantic and Indian Oceansl28. 29]. The paleodepthmagnitude indicate a significant shift in the global carbonof Site 1262 was very close to the modem CCD in thestorage rates. On these timescales, the exchange of carboneastern Atlantic and could have been particularly sensitiveto the rapid deepening of the CCD. Lithology changeswith the organic and carbonate carbon reservoirs is amongfrom brown clay in the late Eocene to light gray fo-the most important fluxes balancing the carbon isotoperaminifer-bearing nannofossil ooze in the early Oligocene,budgetl2. Because fractionation of carbon to the organicindicating that the paleodepth of Site 1262 during the latereservoir is large (~- -22.7 %), a change in the flux of car-Eocene was deeper than the paleo-CCD. Considering thebon to this reservoir could make ocean δ I3C higher. Whystrong dissolution of foraminifers during the early Oligo-would Corg production/burial rates suddenly change dur-the composition of planktonic foraminiferalng the 0i-1 climate transition? Zachos et al. (1996)[fragments, we deduce that the paleodepth of Site 1262suggested two potential reasons. One is that marine fertil-during the early Oligocene is shallower then, but veryity had shifted in response to a dramatic change in oceanclose to the paleo-CCD, suggesting the rapid deepening ofand atmospheric circulation. As the planetary temperature the CCD across the Eocene/Oligocene boundary. At Sitegradient steepened and an ice sheet appeared on Antarc-1265, although an amount of clay minerals are includedtica during the earliest Oligocene, termohaline and at-during the late Eocene, the carbonate contents keep amospheric circulation should have intensified leading torange of 85% - 92%, and therefore, the site reflects thehigher rates of oceanic turnoverl4. In the southern oceans,CCD changes are obviously less than the Site 1262.the strength and persistence of the polar winds would haveThe coarse fraction at Site 1262 can be served as aincreased, thereby enhancing the vigor of surface circula-quantitative indicator for the foraminifer preservationl5].tion and upwelling. and the supply of abundant nutrients,Variations in the coarse fraction during the early Oligo-particularly iron425. The other is that changes in the dis-cene do not appear to be immediate with rapid deepeningtribution and timing of production would have furtherof the CCD; however, they lag behind about 0.5 Ma andenhanced Corg burial rates. Because a greater percentageshow a maximum until 30.0 Ma (Fig. 4). This temporalof detrital organic matter escapes from oxidation regionsrelationship suggests that the shift in carbonate preserva-easily, burial rates would have increased as the balance oftion was triggered by changes in climate. Globally, oceanproduction shifted toward upwelling regionslol. Similarly,carbonate chemistry may have been adjusting to an in-with cooling, production should have become more sea-crease in continental weathering fluxes due to glacial ero-sonal or bloomlike, thus increasing the size of organicsion and the corresponding drop in sea level. Because theaggregates and accelerating the sinking rates of particlesresponse to中国煤化工- Auctivity, and cir-from the surface.culation-drivchould be almost(Lii) CCD changes during late Eocene to early Oli- immediate,TYHCNM H G increase in thegocene transition. The carbonate contents at Site 1262coarse fraction at Site 1262 has a close relationship withare very low during the late Eocene, generally <5%, butcontinental weathering ratesl .abruptly increase to 70% - 90% during the early Oligo-(- i1) Orbitally forced variations in global climateCh月毁据ience Bulletin V0ol.49 No. 20 October 20042195 .ARTICLES(%)796:4:3132'3:34(ka)1004140s (Ma)Fig. 5. Color relecance L* (up) and wavelet analysis sectra (down) during 35 -30 Ma at Site 1262.during the early Oligocene. High-resolution studies of3 Conclusionsmagnetic susceptibility and oxygen stable isotope duringPlanktonic and benthic foraminiferal oxygen andthe Eocene-Oligocene transition at Site 522 documentedcarbon isotopes, carbonate content, and coarse fraction,cyelic variations with periods near those associated withvith high-resolution color reflectance and magneticMilankovitch orbital forcing of climate changel2 0. Thosesusceptibility records during the Eocene-Oligocene transi-orbitally forced variations are also recorded in othertion (35 - -30 Ma) at Sites 1262 and 1265, Ocean Drillingoceans!8]. Because the lithology at Site 1262 during theProgram (ODP) Leg 208 in South Atlantic, reveal thelate Eocene consists almost of clay, it is difficult to recog-climate change occurring near the Eocene/Oligo-nize any climate cyclicity relative to variations in the car-cene boundary. The earliest Oligocene δ O values duringbon cycle. For the early Oligocene, however, we can e-the EOGM event (33.5 - -33.1 Ma) represent the magni-trieve the periodicity of carbonate content variations re-tude of continental ice sheets on east Antarctica and indi-corded by the high-resolution color reflectance.cate the large decrease in both surface and deep waterThe wavelet analysis of the color reflectancetemperature of South Atlantic. The δ B3C records show the34 _- 30 Ma at Site 1262 indicates the periodicities associ-large excursion during the period of EOGM event andated with eccentricity of the Earthi s orbit, 100 and 400 ka,indicate some type of shift in global carbon reservoir,respectively (Fig. 5). The 100-ka eccentricity period of theprobably demonstrating the sudden increase in organiccarbonate variations was also found from other areast 2carbon burial rates and the changes in the distribution aand was suggested to have close relationship with the icetiming of production. At the same time, lithologic compo-volume in high latitudes2I. While the 400-ka ecenticitysition, carbonate content, color reflectance, and coarseperiod was previously reported from the carbon isotopefraction at Site 1262. combined with litology and car-records in the South China Sealb11 and was interpreted asbonate conte中国煤化工about significantthe characteristic process in low latitudes. Though thesechanges nearYHCN M H Gndary, relectingperiods need further study in future, our work confirmsthe abrupt deChanges in car-the orbitally forced variations in global climate during thebonate content revealed from the color reflectance identifythe periodicities associated with eccentricity of the Earth' searly Oligocene.Chinese Science Bulletin Vol.49 No. 20 October 2004219方数据ARTICL .ESorbit (100 and 400 ka), further indicating orbitally forcedography, 1994, 9: 579- -600.global climate variations in early Oligocene.6. Huang, W, Jian, Z, Biuhring, C., The millennial-scale climatefluctuations revealed by the records of color reflectance from ODPAcknowledgements All samples used in this study were provided bySite 1144 in the northern South China Sea, Mar. Geol. & Quatern.the Ocean Dilling Program (ODP), which is sponsored by the u.S.Geol. (in Chinese with English abstract), 2003, 23 (3):5-40.was supported by the National Natural Science Foundation of Chinacene and Oligocene planktonic foraminifers, implications for Oli-(Grant Nos. 40102010 and 40321603), the Shanghai Rising Star Pro-gocene sea- surface temperatures and global ice- volume, Mar. Mi-gram (Grant No.03QE1405 l), the National Key Basic Research Devel-cropaleontol.. 1984, 9: 111- +34.opment Project of China (Grant No. G2000078500), the Excellent YoungTeachers Programs of the Ministry of Education of China, and the8. Millr, K. G., Wright, J. D.. Fairbanks, R. G.. Unlocking the iceODP-China Scientific Committee.house: Oligocene-Miocene oxygen isotopes, eustacy, and marginerosion, J. Geophys. Res, 1991, 96: 6829- 6848.References19. Erez, J, Luz, B., Experimental paleotemperature equation for1. Zachos, J. C.. Pagani, M, Sloan, L. et al.. Trends, rhythms, and ab-planktonic foraminifera, Geochim. Cosmochim. Acta, 1983, 47:errations in global climate 65 Ma to present, Science, 2001, 292:1025- +031.686- 693.20. Miller, K. G, Thomas, E, Late Eocene to Oligocene benthic fo-Zachos, J. C, Quinn, T. M., Salamy, K. A., High- resolution (10*raminifera isotopic record, site 574, equatorial Pacific, Initial Rep.years) deep-sea foraminiferal stable isotope records of the Eo-Deep Sea Drill. Proj, 1985, 85: 771-cene-Oligocene climate transition, Paleoceanography, 1996, 11:21. Rea, D. K, Lohmann, K. C., MacLeod, N. D. et al, Oxygen and251- 266.carbon isotopic records the oozes of ODP sites 752, 754, 756/757,3. Salamy, K. A., Zachos, J. C., Latest Eocene -Early Oligocene cli-eastern Indian Ocean, Proc. Ocean Drill. Program Sci. Results,mate change and Southern Ocean fertility: inferences from sedi-ment accumulation and stable isotope data,1991, 121:229- 240.mentaccumatonand sale iotope data Placogogr,. Palac-climat, Placl 1999 145: 61-7.22. Zachos, J. C, Berggren, W. A., Aubry, M. P. et al, Isotope and4. Robert, C, Diester-Hass, L. Chamley, HH, Late Eocene_Oligocene .trace element geochemistry of Eocene and Oligocene foraminifersoceanographic development at souther high latitudes, from teri-from Site 748, Kerguelen Plateau, Proc. Ocean Dirill, Program Sci.genous and biogenic particles: a comparison of Kerguelen PlateauResults, 1992, 120: 839- 854.and Maud Rise, ODP Sites 744 and 689, Mar. Geol, 2002, 191:23. Zachos, J. C, Lohmann, K. C., Walker,J. C. G. et al., Abrupt cli-mate change and transient climates in the Palcogene: a marine per-5. Barrett, P.. Cooling a continent, Nature, 2003, 421: 221-223.spective, J. Geol, 1993, 101: 193- 215.6. Kennett, J. P., Shackleton, N. J,, Oxygen isotopic evidence for the24. Diester- Haass. L.. Robert, C., Chamley, H, Paleoceanographic anddevelopment of the ps ychrosphere 38 Ma ago, Science, 1976, 260:paleoclimatic evolution in the Weddell Sea (Antarctica) during the513- 515.middle Eocene- late Oligocene, from coarse sediment fraction and7. Miller, K. G., Feigenson, M. D., Kent, D. et al, Upper Eocene toOligocene isotope 87Sr/*Sr, δ 18O, δ "C standard section, Deep- Seaclay mineral data (ODP Site 689), Mar. Geol, 1993, 114: 233 -Drilling Project Site 522, Paleoceanography, 1988, 3: 223- 233.， Diester-Hass, L, Zahn, R., Eocene-Oligocene transition in the25. Diester-Haass, L.. Middle Eocene to early Oligocene paleocean-Southern Ocean: history of water mass circulation and biologicalography of the Antarctic Ocean (Maud Rise, 0DP Leg 113, Siteproductivity, Geology, 1996, 24: 163- +66.689); change from a low to high productivity ocean, Palaeogeogr.9. Moore, T. C. Jr, Rabinowits, P. D.. Initial Reports of the Deep SeaPalaeoclimatol. Palaeoecol, 1995, 113:311-334.Dilling Project 74, Washington: U. S. Govt. Printing Office, 1984,6. Emerson, S., Hedges, J. L., Processes controlling the organic carbon1- 250.content of open ocean sediments, Paleoceanography, 1988, 3:10. Zachos, J. C., Kroon, D., Blum, P. et al, 2004, Proc. Ocean Drill.621- 634.Program Init. Repts., 208 [Online]. Available from htp://wwodp.27. Balsam, w. L., Deaton, B. C, Damuth, J. E, Evaluating opticaltamu.edu/publications/208_ _IR/208ir.htm.lightness as a proxy for carbonate content in marine sediments11. Berger, W. H., Biogenous deep sea sediments: production, presecores, Mar. Geol, 1999, 161: 141-453.vation and interpretation, Treatise on Chemical Oceanography 5(eds. Riley, J. P., Chester, R.), San Diego: Ac ademic, 1976, 265 -8. Berger, W. H., Deep-sea carbonates: evidence for a coccolith lyso-cline, Deep Sea Res, 1973, 20: 917- 921.12. Berger, w. H., Lange, C. B.. Wefer, G., Upwelling history of the9. Peterson, L C., Backman, J. Late Cenozoic carbonate accumula-Banguela Namibia system: a synthesis of Leg 175 results, Proc.tion and the history of the carbonate compensation depth in thewestern equatorial Indian Ocean, Proc. Ocean Drill. Program Sci.13. Nilsen, E. B., Anderson, L. D.. Delaney, M. L., Paleoproductivity,Results, 1990, 115: 467- 507.nutrient burial, climate change and carbon cycle in the westerm30. Mead, G. A. Tauxe, L.. LaBreque, J. J, Oligocene paleoceanogra-equatorial Atantic across the Eocene/Oligocene boundary, Paleo-phy of the South Atlantic: paleoclimatic implic ations of sedimentceanography, 2003, 18: doi: 10.1029/2002PA000804.accumulati14. Jones, A.. Kaiteris, P.. A vacum gasometic technique for rapid andIeocanogr中国煤化工”mesrmoen PBrprecise analysis of calcium carbonate in sediments and soils, J.31. Wang, P.,"YHCNMHGcyclicchangesoftheSediment. Petrol, 1983, 53: 655- 660.ocean carbon reservor, c .ninese Sc1. Bull, 2003, 48 (23): 2536 -15. Bassinot, F. C, Beaufort, L., Vincent, E. et al, Coarse fractionfluctuations in pelagic carbonate sediments from the tropical Indian2548.Ocean: a 1500-kyr record of carbonatedissolution, Palocean-(Received May 17, 2004; accepted June 28, 2004)Ch序野数rience Buletin Vol.49 No. 20 October 20042197 .